Lewisite

CAS RN: 541-25-3

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 143(SRC), determined from a water solubility of 500 mg/L(2) and a regression-derived equation(3), indicates that Lewisite is expected to have high mobility in soil(SRC). Volatilization of Lewisite from moist soil surfaces is expected to be an important fate process(SRC) given an estimated Henry's Law constant of 3.2X10-4 atm-cu m/mole(SRC), derived from its vapor pressure, 0.58 mm Hg(4), and water solubility(2). While the Henry's Law constant for Lewisite indicates volatilization from moist soil surfaces, the rapid rate of hydrolysis may reduce the significance of this fate pathway(4). Lewisite is rapidly hydrolyzed by soil moisture, and minerals present in the soil would speed the process(5). Alkaline soils would neutralize Lewisite(5). However, a specific environmental hydrolysis rate constant was not reported(SRC). Lewisite may volatilize slowly from dry soil surfaces(SRC) based upon its vapor pressure(4). However, Lewisite has been shown to evaporate readily from soil surfaces(6); in one study, 20% of applied Lewisite evaporated from warm soil in the first hour(6); however, evaporation quickly terminates as the Lewisite is converted to lewisite oxide(6,7). No direct information has been found regarding Lewisite biodegradation in soil, however, suggested pathways of microbial degradation in soil include epoxidation of the C=C bond and reductive dehalogenation and dehydrodehalogenation(5).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 143(SRC), determined from a water solubility of 500 mg/L(2) and a regression-derived equation(3), indicates that Lewisite is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon an estimated Henry's Law constant of 3.2X10-4 atm-cu m/mole(SRC), derived from its vapor pressure, 0.58 mm Hg(4), and water solubility(2). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 10 hours and 7 days, respectively(SRC). While the Henry's Law constant for Lewisite indicates volatilization from water surfaces, the rapid rate of hydrolysis may reduce the significance of this fate pathway(4). According to a classification scheme(5), an estimated BCF of 19(SRC), from its water solubility(2) and a regression-derived equation(6), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Lewisite is reported to have a fast hydrolysis rate and is quickly converted to 2-chlorovinyl arsonous acid(7) and HCl(8). Lewisite hydrolysis is dependent on surface area exposure to water vapor and air(9). Water will float on Lewisite and prevent hydrolysis. Lewisite vapor or mist will, through the presence of oxygen and nitrogen, hydrolyze rapidly(9). However, a specific environmental hydrolysis rate constant was not reported(SRC). No biodegradation studies are currently available for Lewisite in aquatic environments(SRC).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), Lewisite, which has a vapor pressure of 0.58 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase Lewisite is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 1.4 and 1.2 days(SRC) for the cis- and trans-isomers of Lewisite, respectively, calculated from the respective rate constants of 1.2X10-11 cu cm/molecule-sec and 1.3X10-11 cu cm/molecule-sec at 25 deg C(SRC) that were derived using a structure estimation method(3). Reaction with ozone will also contribute to Lewisite's atmospheric degradation, but it will not be as rapid as reaction with hydroxyl radicals(SRC). Lewisite has a UV absorption spectrum at 200 to 350 nm(4), which suggests that direct photolysis in sunlight may occur(SRC).
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